This invention relates generally to television signal systems and particularly to television signal systems which utilize digital transmission techniques.
The recent efforts to develop a high definition television system for the United States has created a strong desire to transmit television signals in digital form. A major disadvantage of the digital data format, however, is the lack of redundancy in the data. For example, with conventional analog television signals, a wide variety of noise conditions that may signal dropouts and other transmission problems, can be tolerated. This is because the data or information in the signal is redundant that is, it is repeated often which, to a large extent, permits errors to be integrated out. On the contrary, digital data has no redundancy and the failure to properly detect and decode any portion of the digital data results in that portion of the data being lost. Accordingly, it is very important that low noise components and signals be used in digital signal processing, at least prior to detection of the data, to assure that the true signal level of the digital data (and not noise) is recovered.
In local oscillators in particular, it is imperative that high Q tuned circuits be used. In heterodyning oscillators, mixers and the like, it is essential to avoid phase noise and jitter which give rise to noise in the digital environment and make detection of the correct digital data level very difficult.
The television frequency band in the United States extends from 50 MHz to approximately 800 MHz. In conventional analog tuning systems, a tunable local oscillator produces signals that are mixed with the incoming radio frequency (RF) signals to produce a fixed frequency output signal, e.g. a so-called intermediate frequency (IF), for every channel. In some cable systems, the tuning process uses double conversion in which the RF input signal is initially upconverted to a higher fixed first IF frequency, subjected to appropriate filtering, and subsequently downconverted to the conventional television IF frequency, referred to as a second IF in a double conversion system. A first local oscillator is used for upconversion and a second local oscillator is used for downconversion. The double conversion process, with its main selectivity involving fixed tuning, has the advantage of small channel-to-channel performance variations.
The present invention is directed to a high Q tuning system for a digital television signal. In particular, it is implemented in a double conversion system that includes a synthesizer for producing a tuning signal, in response to a channel input signal, for upconverting the RF signal. The synthesizer includes a plurality of local oscillators that are bandswitched to increase the Q of the tank circuits and thereby improve the noise performance of the system over the television signal tuning range.